1. Field of the Invention
The present invention relates to a wear-resistant member with hard coating which will be used as cutting tools, sliding parts, molding dies, etc.
2. Description of the Related Art
Those members which are used as cutting tools (such as inserts, drills, and end mills), sliding parts, and molding dies usually have hard coating on their surface to meet their requirements for good wear resistance and sliding characteristics. In the past, such hard coating was formed mostly from an oxide, particularly aluminum oxide (or alumina Al2O3), by CVD (chemical vapor deposition). Unfortunately, CVD has a disadvantage of requiring a high treating temperature (exceeding 1000° C.) which causes heat deformation to the substrate; therefore, it cannot be applied to sharp-edged tools and rotating tools. In these application areas, CVD is being replaced by PVD (physical vapor deposition) which permits film formation at comparatively low treating temperatures.
Alumina has more than one crystalline structure. Typical among them is α-type (α-alumina) of rhombohedral system, which has a high melting point and remains stable. Other crystalline structures include γ-type (γ-alumina) of equiaxial system, which has high activity. γ-type is divided into δ-type and θ-type. α-alumina is formed at a higher annealing temperature (oxidizing temperature) above 1000° C. and γ-alumina is formed at a lower annealing temperature ranging from 400 to 700° C. Firing at much lower temperatures gives rise to amorphous aluminum oxide. In addition, γ-alumina transforms into α-alumina at temperatures above 1000° C.
Patent Document 1 discloses a method for forming α-alumina at a process temperature below 500° C. by PVD that employs a Cr-containing source. Patent Document 2 discloses a method for forming γ-alumina on the tool surface by pulsed sputtering.
Patent Document 1:
Japanese Patent Laid-open No. Hei-5-208326
Patent Document 2:
PCT translation No. 2002-544379
Unfortunately, the α-alumina formed at a low temperature from a Cr-containing source does not have a sufficiently high wear resistance. On the other hand, γ-alumina, which consists of smaller grains than α-alumina, has better wear resistance but is unstable in crystalline structure at high temperatures as mentioned above. This instability means that γ-alumina changes into α-alumina at high temperatures, resulting in volume shrinkage and cracking. The tip of a cutting tool encounters a large temperature change from ambient temperature to 1000° C. or above.